Babies who are bottle-fed early on may consume more calories later in infancy than babies who are exclusively breastfed, a study published Monday suggests.

Researchers found that among 1,250 infants followed for the first year of life, those who were bottle-fed during their first six months — whether formula or pumped breast milk — showed less appetite "self-regulation" later in infancy.

The investigators say this so-called "bottle effect" could be one reason that studies have found a correlation between breastfeeding and a lower risk of childhood obesity.

In most research on the question of whether and how breastfeeding might protect against excessive weight gain, the focus has been on the components of breast milk. For instance, breast milk contains certain hormones, including leptin and adiponectin, which could help regulate infants' appetite and metabolism.

But the new findings suggest that the way infants are fed also matters, lead researcher Dr. Ruowei Li, of the U.S. Centers for Disease Control and Prevention, said in an interview. That is, breastfeeding may encourage greater appetite self-regulation in the long term.

In this study, self-regulation was measured when the babies were 7, 9, 10 and 12 months old; mothers were asked how often their babies drank an entire bottle or cup of milk (formula or pumped breast milk).

Li's team found that among infants who had been exclusively breastfed during their first six months, 27 percent always or usually finished their cup or bottle. That compared with 54 percent of babies who had been both breast- and bottle-fed, and 68 percent of those who had been bottle-fed only.

When the researchers accounted for a number of variables — like mothers' weight and education, family income and race — bottle-feeding itself was still linked to lesser self-regulation later on in infancy.

Babies who had had more than two-thirds of their feedings via bottle in early infancy were twice as likely to routinely empty their milk cups as babies who'd had less than one-third of their feedings via bottle.

What's more, the pattern was seen whether those early bottle-feedings contained formula or pumped breast milk.

Li pointed out that obesity is a complex matter, with factors ranging from genetic susceptibility to social and economic factors, to exercise habits all coming into the equation.

But bottle-feeding may be one controllable early-life factor, she said.

It's not certain why breastfeeding might encourage better eating self-regulation. But Li explained that when infants breastfeed, they are in control of how much milk they consume; when parents bottle-feed, they may try to get the baby to empty the bottle each time. It's possible that this interferes with infants' innate ability to regulate their calorie intake in response to internal "appetite cues."

Many women who breastfeed use a breast pump at some point, particularly after they return to work. Li suggested that parents who use bottles pay attention to cues that their baby is full — such as trying to push the bottle away with the tongue or shaking the head to move away from the bottle.

"You don't have to push until the bottle is empty," she said.

Further research, according to Li, is needed to see whether early bottle-feeding is associated with appetite self-regulation in older children as well.